1. Field of the Invention
The present invention relates to an apparatus for effecting time-space conversion on a light signal changing at ultra high speed.
2. Related Background Art
Recent progress is remarkable in technology for producing ultrashort pulses, so that stable pulses of less than a pico second can be obtained at present. Such ultrashort pulses may be used as light source for time resolved spectroscopy, for nonlinear spectroscopy, or for high-speed optical communication.
A waveform of naturally existing light or artificially produced light includes various useful information. Precise observation of the waveform enables deeper recognition and more precise understanding of natural phenomena. There are two types of methods for measuring a waveform of a light signal, roughly classified as follows.
(1) Method utilizing the linear photo-electronic conversion
(2) Method utilizing the nonlinear optical effect
The former is a method for measuring a waveform in which a light signal is converted into an electric signal or a charge signal by a photodetector utilizing the photoelectric conversion and then the detected signal is measured on an oscilloscope. In this method, the photodetector may be a photodiode, a photo tube, a photomultiplier or a streak tube. Among the alternatives listed, a streak camera using the streak tube is highest in speed, and presently commercially available streak cameras have a time resolution of 600 fsec.
The latter is a method for measuring a waveform of light to be measured in which a suitable delay is given between light to be measured (signal light) and probe light, in which the two types of light are combined on a nonlinear crystal to obtain a harmonic light, and in which a waveform is obtained from the intensity of harmonic light with the parameter of delay amount. This method is substantially a sampling method for measuring a waveform of repetitive waves, but can also be used for real time waveform measurement of single pulse light by spatially expanding the light to be measured and the probe light and superimposing them on each other while giving spatially different delay times therebetween (Norimoto Kawanishi et al., "Measurement of ultra-high-speed light pulse by optical sampling technique", research materials, instrumentation research group at Institute of Electrical Engineers of Japan, IM-89-15, May 30, 1989; Japanese Patent Publication No. 2-259435; Kazuhiro Ema et al., "Ultra-high-speed light waveform measuring method using waveform shaping, and its applications", annual reports of general research institute, vol. 50 (1991), the University of Tokyo, Department of Technology ).